The present invention relates to a new and improved circuit arrangement for superposing or impressing starting or start-up conditions upon an electrical operating means-simulator composed of RC-elements wherein the simulator network is charged by a charging current wich is functionally dependent upon the operating current to a charging voltage representative of the heating-up or thermal behavior of the electrical operating means and the charging voltage appears at a low-ohmic output of a charging state-storage device for controlling an electronic operating means-protection relay.
In the case of electrical simulators for operating means or equipment, especially motors and transformers, there is simulated by means of the capacitance of the first RC-element the copper-thermal capacity, by means of the capacitance of the second RC-element the iron-thermal capacity, and by means of the resistances of the RC-elements the corresponding thermal transfer resistances of the electrical operating means. With such an electrical simulator and by supplying the RC-elements with a charging current, the current intensity of which is proportional to the square of the operating current intensity, and with proper dimensioning (time-constants), it is possible to detect the thermal behavior of the operating means brought about by electrical losses in such a satisfactory manner that the charging voltage of the simulator sufficiently accurately reflects the momentary temperature of the operating means for the control of the subsequently connected protection relay.
In order to obtain greater accuracy of the simulator with an acceptable expenditure in electrical components or equipment, the charging current is advantageously delivered in a clocked fashion via a switch and the low or bottom ends of the simulator capacitors are at a direct-current potential in order to prevent leakage currents at the trigger point. In this regard attention is directed to Swiss Pat. No. 540,587, the disclosure of which is incorporated herein by reference. For the purpose of accommodating the first simulator capacitance to the thermal capacity of the motor which is to be momentarily protected the effective capacitance of the capacitor is advantageously influenced with the aid of electronic means, for instance as taught in the aforementioned Swiss Pat. No. 540,587, and also the different cooling behavior of the object to be protected when the same is running and at standstill is taken into consideration by advantageously electronically influencing the simulator resistances. In the case of failure of the supply voltage or power supply for the protection relay, the circuits for influencing the effective capacitance and the effective thermal resistances become inactive and the capacitors discharge in the direction of the low or bottom ends which are at higher direct-current potential or voltage during normal operation, so that upon the re-appearance or recurrence of the supply voltage the simulator voltage corresponds to a heated-up object which is to be protected. Moreover, the electronic switch, through the agency of which there is delivered the charging current of the simulator, becomes conductive when there is a breakdown in the supply voltage, so that there also occur discharge operations thereacross. In the case of a starting charging voltage which is too high in relation to the actual temperature of the electrical operating means there can result too early shutting-down of the operating means by the protection relay, and in the case of a starting charging voltage which is too low there can result a shutting-down of such operating means which is too late by means of the protection relay. For a permissible mode of operation of the electrical operating means-protection relay, measures therefore must be taken to insure that after each interruption in the supply voltage there are present the proper starting conditions at the electrical simulator. This can be basically obtained in two different ways: Either the circuit arrangement containing the simulator is designed such that after every interruption in the voltage supply there are automatically present the proper starting conditions, or after each interruption in the voltage supply the starting conditions are impressed or superposed at the simulator. In the first case there is necessary for the current supply of the circuit arrangement a separate current source, the provision of which generally however is undesired. In the second case, and with which the present invention is concerned, there is present the problem of having to distribute a starting condition to two simulator capacitors. A correct distribution of a starting or start-up condition to two (or more) simulator capacitors, however, is not possible at least with the means which are still economically feasible for such circuit arrangements.